It has to do with cleavage. Pressing boobs together makes their volume increase, and push them higher which creates more cleavage. They look bigger. And cleavage is something that guys find very attractive. I'm not sure about this next part since I'm not a girl but I believe that pressing boobs together and making them higher possibly makes them feel less heavy for the girl. I think this because having something hanging down tends to push the rest forward and down. That causes more back pain. Make sense? So, that's what I think.
I think they're both fine, but the second picture is a little more attractive.
Cleavage emphasizes the curvature of the woman's body and subtly draws attention to other areas of squishy roundness, like the buttocks and vulva. According to a book I read on body language, the average man cannot tell the difference between a woman's buttocks and her two breasts squished together if the nipples aren't showing.
Mine are usually left looking normal like on the left but if I pull my boobs up more in my bra or wear a push up bra, they look like that on the right. I mainly do that when I wear vest tops to improve my overall figure because my vest tops are tight fitting. It is done for male attention and/or to look more feminine with bigger boobs.